Cant we use the same IP address allocated for a UE to connect multiple PDNs? Why different IPs for each PDN?

No, IP address shall be different for each PDN connection because there should be ONLY ONE default bearer per PDN.

TS 23.401

5.3.1 IP address allocation
The IP address allocated for the default bearer shall also be used for the dedicated bearers within the same PDN connection. IP address allocation for PDN connections, which are activated by the UE requested PDN connectivity procedure, is handled with the same set of mechanisms as those used within the Attach procedure.
 
* Default Bearer: The EPS bearer which is first established for a new PDN connection and remains established throughout the lifetime of the PDN connection.

I hope this will be helpful for you. (source: http://lteuniversity.com/ask_the_expert/f/59/t/2435.aspx)

Why multiple IP addresses are needed, if at all. This is more of a practical implementation discussion. IF the UE asks to be connected to two different APNs (Access Point Name) (please note the emphasis on the If) AND IF the Service Provider has implemented the APNs on two different PGWs then the UE will be setup with two PDN Connections and each of the PGWs will give the UE a different IP address. Remember, as far as the external routers are concerned the packets are coming from a PGW and the IP addresses are at the PGW and the packets destined for the UE must be sent to the PGW. It is the PGW that knows which EPS bearer to tunnel these packets through. Does the Service Provider have to configure the APNs in two separate PGWs - NO. But, for larger networks and as an example networks who serve multiple end user clients the Service Providers are almost required through their Service Level Agreements (SLAs) to take this approach.

can you explain me GUTI concept and ghummi concept in lte attach 

can you explain usim and usim secret key

Refer to the LTE Authentication section of Netmanias  https://www.netmanias.com/en/?m=view&id=techdocs&no=5902. For details of the Auth algorithms refer to spec 33205, 33206 and 33208

That helped me in understanding the concept. Thank's!!